Sure.
Yes. If there's some way to tell whether a particular page is in a lazy
mapping then that would help, since we could easily tell whether we need
to do the whole shootdown thing. I would expect the population of
lazily mapped pages in the whole freepage pool to be pretty small, but
if the allocator tends to return the most recently freed pages you might
hit them fairly regularly (shoving them at the other end of the freelist
might be useful).
That's right. The hypervisor doesn't trust the guests, so it prevents
them from getting into a state where they can do bad things.
Sounds good.
For now I've proposed a patch to simply eagerly vunmap everything when
CONFIG_XEN is set. It certainly works, but I don't have a good feel for
how much of a performance hit that imposes on XFS. A slightly more
subtle change would be to test to see if we're actually running under
Xen before taking the eager path, so at least the performance burden
only affects actual Xen users (and I presume xfs+xen is a fairly rare
combination, or this problem would have turned up earlier, or perhaps
the old xenified kernels have some other workaround for it).
J
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